Most water contains alkaline earth metal cations, such as barium, strontium. calcium. and magnesium. and anions such as sulfate, bicarbonate, carbonate oxa-ate phosphate, silicate, flouride, etc. When combinations of these anions and cations are present in concentrations which exceed the solubility of their reaction products, preecipitates form until their product solubility concentrations are no longer exceeded. For example, when the barium ion and sulfate ion exceed the solubility of the barium sulfate reaction product, a solid phase of barium sulfate will form as a precipitate.
Solubility product concentrations are exceeded for various reasons, such as evaooration of the water phase, change in pH, pressure or temperature, and the introduction of additional ions which can form insoluble compounds with the ions already present in the solution.
As these reaction products precipitate on the surfaces of the water-carrying or water-containing system, they form adherent deposits or scale. The scale prevents effective heat transfer, interferes with fluid flow, facilitates corrosive processes, and harbors bacteria. Scale is an expensive problem in many industrial water systems, in production systems for oil and gas, in pulp and paper mill systems, and in other systems, causing delays and shutdowns for cleaning and removal.
Scale-forming compounds can be prevented from precipitating by inactivating their cations with chelating or sequestering agents, so that the solubility of their reaction products is not exceeded. Also, certain substances are known to function as scale or precipitation inhibitors in a potentially scale-forming system at a markedly lower concentration than that required for sequestering the scale cation. Such substances are known as "threshold active" compounds.
Once through and recirculating, cooling water systems are subject to the formation of scale deposits. Waterside problems encountered in boilers and steam systems include the formation of scale and other deposits, corrosion and foam. Scale and other deposits on heat-transfer surfaces can cause loss of the thermal efficiency of the boiler and can make the temperature of the boiler metal increase. Under scaling conditions, temperatures may go high enough to lead to failure of the metal due to overheating.
Barium and strontium sulfate scale deposits present a unique and sometimes "unsolvable" problem. Under most conditions, these sulfates are considerably less soluble in all solvents than any of the other commonly encountered scale-forming compounds. It is generally acknowledged that barium sulfate scale is almost impossible to remove by chemical means. Consequently, barium sulfate deposits must be removed mechanically or the equipment, pipes, etc., containing the deposit must be discarded.
The incidence of barium sulfate scale is worldwide, and it occurs principally in systems handling subsurface waters. The barium sulfate scale problem is of particular concern to the petroleum industry since increasing volumes of water are produced with petroleum and more petroleum is produced by the waterflooding method of secondary recovery. The scale may occur in many different places, including production tubing, gathering lines, meters, valves, and in other production equipment.
Barium sulfate scale may also form within subterranean formations such as in disposal wells. Scales and deposits can be formed to such an extent that the permeability of the formation is impaired resulting in lower flow rates, higher pump pressures, and ultimately abandonment of the well.
An excellent review of the state-of-the-art is presented in the book by Jack C. Cowan and Donald J. Weintritt entitled "Water-Formed Scale Deposits," 1976, Gulf Publishing Co., Houston, Tex. U.S.A. An informative article by O. J. G. Vetter entitled "How Barium Sulfate Is Formed: An Interpretation" was published in the December, 1975, issue of the Journal of Petroleum Technology at pages 1515-1524.
Thus there is a great need for a chemical system which will remove or enhance the removal of barium sulfate scales, deposits, or crystals from various surfaces, and which will inhibit or prevent the formation of barium sulfate when the chemical system is present under normal precipitate-forming conditions.
Accordingly, it is an object of this invention to provide chemical compositions which are effective in solubilizing barium sulfate scales, deposits, crystals, and the like, and in preventing the formation of barium sulfate.
It is another object of this invention to provide a method of solubilizing barium sulfate thus enabling the removal of adherent barium sulfate scales, deposits, crystals, etc. from surfaces, and a method of preventing the formation of barium sulfate.